Effect of spatial coherence on photolithographic phase mask performance

Abstract

Since Marc Levenson’s introduction of phase masks to photolithography [1], there have been numerous improvements in their design and fabrication [2-6]. One issue not yet considered is the effect of illumination source spatial coherence on the effectiveness of phase masks in improving resolution and focal depth. As the illumination wavelength has grown shorter (to accommodate the classical resolution limit Δ x ≅ λ f D ≅ λ N A , sources have become more spatially coherent [7], and with the use of excimer laser illumination we approach complete spatial coherence. It has generally been assumed that increased spatial coherence improves the performance of phase masks, since it enhances the destructive interference between adjacent (opposite phase) features. Although this is true for periodic patterns, we show here that more complex images suffer a trade-off between the contrast enhancement of coherent alternating features and the larger effective aperture of incoherent imaging. This effect is particularly pronounced in complex 2-D images where phase conflict is a problem.